Sight unseen, solely from my experience, you're probably moving too fast, leaving enemies at your rear, or too slow, allowing them to build up and attack where you cannot retreat. You only want to spend a turn or two, maybe three, recruiting, then push North. Be sure to watch the time of day and terrain and try to use those either to your advantage or, at least, deny your opponent an advantage.

If you are just beginning wesnoth I recommend playing it on "normal" difficulty. You might be running the scenario just fine but are just lacking the experience deal with the extra units hard is adding. If you insist on staying on hard, make sure to deny avenues that let more than two enemy units attack frail or injured units, understand wesnoth is rng so a single digit chance to die is usually not worth the risk, and importantly, sometimes you must sacrifice an unit. Putting three units in risk for one is not a great trade off. Also, if this is the second scenario it will be better to use lvl 1 units that are close to leveling over lvl 2 units. The full heal on level-up will in theory give you more damage you can soak up and the lvl 1 units will not drain your upkeep as fast as lvl 2s. As for loyal unit survival rate, do your best to keep it high. Loyal units will pay for themselves quite a few times over so they are worth sacrificing other units for in more cases than normal.

How many units are you recalling from the previous scenario? I recalled four units, only one of which was level 2, and recruited nine spearmen. I lost three units during the scenario, one loyal and two of the new recruits.

How many units are you recalling from the previous scenario? I recalled four units, only one of which was level 2, and recruited nine spearmen. I lost three units during the scenario, one loyal and two of the new recruits.

Imo the "official" walkthroughs are useless for the higher difficulties. Almost everything in them seems aimed at easy and medium.